@afzalap163 I think the problem you are facing due to some reasons
First part

MAILGUN_API_KEY = ‘This is the private API key which you can find on your Mailgun Dashboard’
SANDBOX_URL= ‘You can find this on your Mailgun Dashboard’
SENDER_EMAIL = ‘This would be test@your SANDBOX_URL’
RECIPIENT_EMAIL = ‘Enter your Email ID Here’
API_KEY = ‘This is your Bolt Cloud account API key’
DEVICE_ID = ‘This is the ID of your Bolt device’
you have to check that the above information enter by you is correct and remember you have to only enter mail adress which was registered on mailgun dashboard.
Second part can be due to code
so i am sharing screenshot for both first and second part so you can get idea that what mistake might you have done